1. Field of the Invention
The present invention relates to capacitive-type fuel quantity measurement probes and more particularly to a method and means for automatic real-time calibration of remotely-located fuel quantity measurement probes of the capacitive type.
2. Problem to be Solved
Capacitive-type fuel quantity measurement probes are typically constructed using two or more concentrically arranged aluminum cylinders that act as capacitor electrodes. Fuel is disposed between the electrodes and the fuel quantity is determined by measuring changes in the probe capacitance. The capacitance changes are proportional to the fuel level present and the dielectric properties of the fuel. Currently, there is an increasing demand for high-accuracy fuel quantity measurement systems, e.g., +/-0.75% of full scale system accuracy at empty, along with the requirement that each component of the system, e.g., probes, fuel quantity measurement electronics, and harnesses, be fully interchangeable and not require calibration or adjustment when installed on a vehicle such as an aircraft.
Since manufacturing tolerances for aluminum cylinders and associated insulating spacers and terminal blocks used in probe construction typically introduce an error of 1% or more in the expected probe "dry" capacitance, to guarantee interchangeability with all other components of the system each probe must be individually factory-trimmed after manufacture to a tight tolerance, e.g., +/-0.2%.
Prior Art
The conventional solution to probe trimming is to add a small variable trimmer capacitor in parallel with the probe electrodes and adjust it to a pre-set, "total padded" capacitance design value. For example, a 47.0 pF probe with a +/-0.5 pF manufacturing tolerance is padded with a 1.0 - to - 5.0 pF variable trim capacitor that is adjusted so that the combined probe/trim capacitance always equals a "total padded" value of 50.0 pF.
A typical DC probe for fuel measurements is shown in the schematic of FIG. 1a if the capacitor C.sub.fhc is ignored. The C.sub.probe capacitor has a parallel leak resistor R.sub.leak and is excited by an AC waveform, e.g., a 30 KHz sine wave at 15 volts peak, input on line 10. Changes in probe capacitance correspond to changes of fuel level and fuel dielectric constant on the C.sub.probe capacitor. The output current from C.sub.probe is rectified by a reverse-biased diode D.sub.1 and a forward-biased diode D.sub.2 to prowide a pulsating negative DC current "return" signal back to the remotely-located probe measurement electronics 30 on output lines 11 and 12. Conversely, diode polarity of D.sub.1 and D.sub.2 can be reversed in common practice with a resultant positive DC current on output lines 11 and 12. A capacitor C.sub.trim is added in parallel with C.sub.probe to allow adjustment to the final "total padded" capacitance required.
A typical DC probe can be converted to a DC FHC (Full-Height-Compensated)-style probe as shown in FIG. 1a by including the C.sub.fhc capacitor coupled in series with C.sub.probe to provide fuel density correction over the full wetted length of the probe in conjunction with an inverse-FHC physically-tapered inner electrode. The DC FHC probe assembly may be trimmed to a final "total padded" capacitance by the C.sub.trim capacitor which is connected in parallel with the C.sub.fhc and C.sub.probe network. The physical C.sub.fhc capacitor may be modeled in software ("software FHC"), if desired, eliminating the need for a physical component.
Equivalent AC versions of the DC probe elements can also be constructed by replacing the diodes D.sub.1 and D.sub.2 with a shielded coaxial cable having a center conductor 12 and a shield wire 11 as shown in FIG. 1b. The shielded coaxial cable (11, 12) between the probe assembly and remote measurement electronics 30 may be any convenient length, such as 30 feet.
In all probe embodiments, the R.sub.leak resistor represents the resistive component that is effectively in parallel with C.sub.probe due to water, algae, fuel contamination, or conductive additives in the fuel. Since R.sub.leak creates an undesirable error source, remotely-located fuel measurement circuitry is used to "flag" a condition where R.sub.leak appreciably effects accuracy. A typical technique involves switching the probe excitation frequency from "F" to "2F" and observing that the reactive component (C.sub.probe) doubles in current, while R.sub.leak remains constant. In so doing, the R.sub.leak component may be effectively detected.
The disadvantages of the conventional trimming approach include:
Variable trim capacitors are fragile, expensive, and may shift in value when potted;
Potting and complete sealing of the trimmer capacitor is a requirement if the capacitor is located in an area subject to fuel intrusion;
The trimming process is tedious, as a small trimmer must be adjusted in the factory by a technician while watching a precision capacitance meter, and interactive trim/retrim sequences may be needed, as stray capacitances may change when a screwdriver is inserted or removed from the trimmer area; and
The capacitance trim value may shift after manufacture due to vibration, aging, or fuel contamination effects.
Objects
It is therefore an object of the present invention to provide a quick, permanent calibration system for each raw manufactured capacitive-type remotely located fuel probe.
It is another object of the invention to achieve trim accuracies of +/-0.1 pF or better in capacitive-type fuel probes.
It is a further object to achieve such trim accuracies with no shift in trim value due to vibration, aging, or fuel contamination effects.
It is a particular object to achieve the foregoing improved calibration and trim accuracies by merely using a single, inexpensive fixed "signature resistor" and a fixed, non-critical DC blocking capacitor.